Influenza Vaccine Efficacy: As Low as 31%?

By Amesh A. Adalja, MD, FACP, February 22, 2013

The cornerstone of influenza prevention—at both the population and individual levels—has been the influenza vaccine. The current vaccination strategy involves yearly immunizations against ever-changing influenza strains with vaccines that, hopefully, are ideally matched to circulating strains of virus. However, even when optimally matched, the standard trivalent influenza vaccine (TIV) has an efficacy of approximately 60% in preventing influenza infection. In the elderly and other specific segments of the population, though, the flu vaccine may have no proven benefit.1 A recent study by Ohmit and colleagues adds to the mounting evidence that our current influenza vaccines are deficient.2

Study Design and Results

In this study, the researchers recruited 328 households in Michigan for a total of 1441 subjects, 839 of whom were children. To quantify efficacy, the study took place during the 2010-2011 influenza season when the vaccine was well matched. Of the 60% of subjects who had documented influenza vaccination for the season, 88% received TIV, and 12% received the live-attenuated influenza vaccine (LAIV). The vast majority of LAIV recipients (96%) were children. Vaccinated and nonvaccinated study participants were followed throughout the season, and respiratory specimens were obtained from those who developed symptoms of upper respiratory tract infection. These specimens were then tested via PCR for the presence of influenza virus.2

Of the 983 specimens collected, 130 tested positive for influenza. Strains detected included H3N2 (59%), H1N1 (20%), and influenza B (34%). The risk of influenza in a vaccinated subject was 8.5%; in an unvaccinated subject, it was 8.9%—a difference that was not statistically significant.

Stratifying cases with respect to community or household acquisition, based on exposure to index cases among the cohort, the vaccine was found to have 31% efficacy in preventing community-acquired infection (with a 95% confidence interval that spanned -7 to 55%). When these cases were further stratified based upon receipt of the influenza vaccine during the prior season, it was found that those who were not vaccinated in the season prior exhibited a 62% vaccine efficacy rate. This result suggests that repeated influenza vaccination may have a deleterious effect. There was no evidence that influenza vaccination prevented household acquisition of the virus.2

A Game-Changing Vaccination Strategy

This study, coupled with several others, highlights the importance of heeding Dr. Michael Osterholm’s recent call for “game changing vaccines.”3 Such an ideal vaccine would provide high vaccine efficacy across all subgroups with no need to switch composition annually. Moreover, this study’s finding of blunted vaccine response with repeated annual immunization provides additional evidence in support of the notion that yearly vaccination may not be the best strategy.

Given that the study did not include a large number of recipients of LAIV or the high-dose formulation of TIV, the study’s results may not apply fully to those vaccines. Similar studies with those vaccines would be informative.

While we remain years away from an ideal vaccine,4 Ohmit and colleagues’ research emphasizes the goal is worth reaching.